An ionic activity-measuring apparatus utilizing an ion selective electrode is widely employed for analyzing ionic components in a liquid sample such as a whole blood sample or a serum sample.
U.S. Pat. No. 4,571,293 (which corresponds to EP 0 160 997 B1) discloses an ionic activity-measuring apparatus using an ion selective electrode which is illustrated in FIG. 1 of the drawings attached to this specification.
In FIG. 1, the ionic activity-measuring apparatus comprises a non-electroconductive support 11, a pair of electrodes each of which comprises a silver metal layer 12a, 12b and a silver halide layer 13a, 13b, a common electrolytic material layer 14, a common ion selective membrane 15, and a common non-electroconductive cover sheet 16 having a pair of openings 17a, 17b for receiving and keeping a sample solution and a reference solution, respectively, each opening being placed above each electrode unit, and having thereon an a bridge member 18 for electrically bridging the sample solution received in one opening 17a and the reference solution received in another opening 17b. Each of the silver metal layer 12a, 12b has an exposed surface 10a, 10b, respectively. By placing probes of a potentiometer 19 on these exposed silver metal surface 10a, 10b, the produced electric potential difference can be measured.
FIG. 2 illustrates an industrially employed process for preparing the ionic activity-measuring apparatus of FIG. 1.
In industry, the ionic activity-measuring apparatus of FIG. 1 is generally manufactured in a mass scale, by the steps of:
(1) preparing a longitudinal sheet composed of a non-electroconductive sheet 11 and a silver metal layer 12 deposited or laminated on the non-electroconductive sheet 11;
(2) making a linear scratch 21 on the silver metal layer 12 under the condition that the scratch is extended in the longitudinal direction and reach the non-electro-conductive sheet 11, to divide the silver metal layer 12 into two silver metal portions 12a, 12b, and covering each silver metal portion 12a, 12b on its side areas with a polymer material layer 20a, 20b, leaving uncovered area in the form of a stripe (or belt) on the silver metal portion 12a, 12b; 
(3) halogenating the silver metal portion 12a, 12b in the uncovered areas to form silver halide layers 13a, 13b, respectively, on the surface of the silver metal portion in the uncovered area;
(4) forming an electrolytic material layer 14 on the scratch and the silver halide portions;
(5) peeling the polymer material layer 20a, 20b off from the silver metal layer to produce an exposed silver metal area 10a, 10b, and placing an ion selective membrane 15 on an electrolytic material layer 14; and
(6) finally placing a non-electroconductive member 16 having two openings 17a, 17b on the ion selective membrane 15 and then placing a bridge 18 on the member 16 to connect the openings 12a, 17b. 
The above-described industrial method is advantageous for manufacturing a great number of ion selective electrodes in a mass scale.
The ion selective electrode can measure an ionic activity of H+, Li+, Na+, K+, Mg2+, Ca2+, Cl−, HCO331 , or CO32−, by employing an appropriate ion selective membrane.
U.S. Pat. No. 4,789,435 describes an ion selective electrode assembly comprising plural ion selective electrodes for analyzing plural ionic components such as Na+, K+, and Cl−, simultaneously. In the assembly, one of plural ion selective electrodes has an ion selective membrane differing from that of other ion selective electrode in chemical composition.